1. Field of the Invention
This invention relates to a light emitting device and to a method of making such a device. The invention is particularly directed to a device which employs luminescent semiconductor properties.
2. Discussion of Prior Art
There has been much research recently into visible electroluminescence from porous silicon, which is a sponge-like structure made by first anodising and then etching a silicon substrate. Three papers which discuss the present state of the art are as follows: L. T. Canham, "Silicon Quantum wire array fabrication by electrochemical and chemical dissolution of wafers"; Appl. Phys. Lett. 57 (10), 3rd Sep. 1990, page 1046; Nobuyoshi Koshida and Hideki Koyama, "Visible electroluminescence from porous silicon", Appl. Phys. Lett. 60 (3), 20th Jan. 1992, page 347; and Volker Lehman and Ulrich Gosele, "Porous Silicon: Quantum sponge structures grown via a self-adjusting etching process", Adv. Mater. 4 (1992) No. 2, page 114.
The basic prior art light emitting device is disclosed in the aforementioned paper by Koshida and Koyama. It consists of a diode structure made from a substrate of p-type silicon, on top of which is formed a porous layer, typically 0.2 .mu.m to 1.0 .mu.m thick. Electrodes are placed both on the porous layer and on the underside of the substrate so that an electrical bias potential can be applied to the diode. One of the electrodes, that on the porous layer, is made of semi-transparent material so that light generated within the diode structure may be emitted.
The porous layer is formed by subjecting the top surface of the substrate to anodisation. This is believed to produce a porous layer comprising an array of columns or wires of low dimensionality, vertical to the surface, and separated by holes or spaces and wherein each column comprises silicon embedded in silicon oxide. Generally the porosity of the layer is increased by subsequent etching. An etchant is used which thins the columns by chemical dissolution, with a resultant increase in the size of the spaces between the columns. Light is generated within the diode structure in response to electrical bias. There is currently some uncertainty in the scientific world as to how the structure emits light.